Process for the pyrolytic conversion of a polymeric precursor composition to boron nitride

ABSTRACT

The disclosure is directed to polymeric B-aminoborazene compounds suitable for pyrolytic conversion to boron nitride. The B-aminoborazene compounds are preferably mixed with an organic solvent and a cross-linking agent to form a polymeric gel. The polymeric gel is then pyrolized to form boron nitride. The polymeric gel is useful to coat various forms and materials.

BACKGROUND OF THE INVENTION Cross-Reference to a Related Application

A related application entitled PRECURSORS FOR BORON NITRIDE COATINGS,U.S. Ser. No. 07/312,956, to Paine et al., is being filed concurrentlyherewith, and the specification thereof is incorporated herein byreference.

Field of the Invention

This invention relates to monomer and polymer precursors useful for theformation of boron nitride articles and coatings.

Description of the Related Art

Non-oxide ceramic materials, such as borides, carbides and nitrides, inone or more crystalline modification, are known for their highmechanical strength, hardness, corrosion resistance, oxidationresistance and thermal shock stability. One commercially importantexample is provided by boron nitride, BN. Boron nitride has severalcrystalline modifications with the common hexagonal (α) form beingisostructural with graphite and the cubic (β) form being isostructuralwith diamond. Despite these structural similarities, boron nitride hasmuch more favorable physical and chemical properties in comparison tocarbon. For example, boron nitride has a high melting point (3000° C.),high anisotropic thermal conductivity, excellent dielectric properties,low chemical reactivity and high temperature semiconductorcharacteristics. Applications for hexagonal boron nitride includecrucibles for metal evaporation, transistor heat sinks, nuclear reactorcontrol rods, high temperature (800° C.) solid lubricants, metalcorrosion resistant coatings and ceramic fiber coatings.

Boron nitride has been typically prepared in the prior art by hightemperature pyrolysis (900°-1200° C.) of simple boron and nitrogencontaining materials, e.g. B(OH)₃ and urea. More recently, boron nitridehas also been prepared by chemical vapor deposition (CVD) of mixtures ofBCl₃, BF₃, or B₂ H₆ with NH₃. In each case, the α-form of boron nitrideis normally obtained, and it is most often produced as a powder.Increasing demands for fibers, films, foams, etc., require newapproaches to obtain α- and β-boron nitride.

There is currently a large effort underway in the United States andabroad in the utilization of polymer pyrolysis as a route to solid statematerials. This concept, coupled with sol-gel and aerogel processingtechniques, which were developed for silica glass processing, hasallowed for the generation of new forms of well known solid statematerials, as well as new families of materials. In particular, newoxide glasses, carbide fibers, silicon nitride and boron carbide havebeen obtained from pyrolysis of appropriate polymers. However, verylittle effort has been devoted to preparing boron nitride by polymerprecursor techniques since very few characterized polymers containingboron and nitrogen have been reported. A brief outline of some pertinentwork which is in published literature and/or in patents is providedbelow.

Several efforts were made to prepare polymeric boron-nitrogen containingcompounds in the early 1950s through 1960s. A good deal of the work thatwas published involved borazene and substituted borazenes as the monomerspecies. Harris, J. Org. Chem., 26, 2155 (1961), reported N-B couplingof two borazenes, but no polymers were described. Laubengayer, et al.,J. Am. Chem. Soc. 83, 1337 (1961), reported on the thermal decompositionof the parent borazene H₃ B₃ N₃ H₃. It was suggested that polymericintermediates were formed, but they were not characterized. Wagner, etal., in "Synthetic High Polymers," Chemical Abstracts, 38349W, Vol. 66,p. 3685 (1967); "Borazine Polymers. B-N Linked Borazine Rings andPolyborazylene Oxides," Inorganic Chemistry, Vol. 1, pp. 99-106 (1962),and U.S. Pat. No. 3,288,726, entitled B-N LINKED BORAZENE DERIVATIVESAND THEIR PREPARATION, described the pyrolytic dehydrogenation ofsubstituted borazenes and resultant formation of N-B coupled borazeneswhere the coupling chemistry directly linked B and N atoms in two rings.Wagner also described coupling of two borazene rings via an exo oxygenatom giving a B-O-B bridge. In the first case, it was proposed that tenrings could be coupled while, in the latter case, it was suggested thattwo to 23 rings could be coupled. In the '726 patent, a good deal ofcross-linking chemistry involving substituted borazenes was described.Horn, et al., in U.S. Pat. No. 3,345,396, entitled ORGANO-SUBSTITUTEDBORAZINES; No. 3,392,181. entitled CYCLIC BN-COMPOUNDS; and No.3,382,279, entitled PROCESS FOR THE PRODUCTION OF SILICON-CONTAININGN:N':N"-TRIORGANO-B:B':B"-TRIHYDRIDO-BORAZOLES; reported more complexpolymerization chemistry involving large organic coupling agents. A.Meller, in Monatsh. Chem. 99, 1670 (1968), reported reactions of aminosubstituted borazenes with diborane, which led to cleavage of the aminogroup on the borazene. No mention was made of the use of the polymersdescribed in these reports for boron nitride precursors and no extensivehigh temperature pyrolysis chemistry was examined.

Patterson, U.S. Pat. No. 3,321,337, entitled PROCESS FOR PREPARING BORONNITRIDE COATINGS, described an ambient pressure chemical vapordeposition process for α-boron nitride deposition on metals usingB-trichloro borazene, Cl₃ B₃ N₃ H₃.

Taniguchi, in Japan Kokai 76 53,000 (Chem. Abstr. 85, 96582v (1976),reported the formation of filaments and films of boron nitride byextrusion and pyrolysis of a polymer obtained by heating (H₂ NBNC₆ H₅)₃.No further details of the formation, characterization and processing ofthe polymer have appeared.

In 1978, Meller, et al., in Z. Naturforsch, 88b, 156-158, reportedreactions of B-2-alkyl,-4,6-dichloro,N-1,3,5 trimethylborazines withhexamethyl disilizane, which produced polymers which were only partiallycharacterized. Pyrolysis chemistry of the polymers was not described.

In 1984, Paciorek, et al., in Polym. Prepr. (Am. Chem. Soc., Div. Polym.Chem.) 25(1), 15 (1984) (Abstr.), reported condensation reactions ofseveral substituted borazenes. All of this chemistry presumably involveddirect ring-ring coupling (B-N bonds). Subsequently, the same group,Paciorek, et al., in Chemical Abstracts, 104, 211726v. (Abstr.); U.S.Pat. No. 4,581,468, entitled BORON NITRIDE PRECERAMIC POLYMERS; and"Boron-Nitrogen Polymers. I. Mechanistic Studies of Borazine Pyrolyses,"Journal of Polymer Science. Vol. 24, pp. 173-185 (1986); discussed theuse of these polymers as preceramic polymers, and they described somelimited pyrolysis chemistry. They claimed that boron rich boron nitrideswere obtained as black solids.

Bender, et al., in Ceram. Eng. Sci. Proc. 6, 1171 (1985), incollaboration with Paciorek, examined further details of the pyrolyticchemistry of substituted borazenes including the monomer (H₂ NBNC₆ H₅)₃utilized by Taniguchi. In contrast to Taniguchi, they observed theformation of amorphous non-stoichiometric (boron-rich) materials fromthis precursor. Other borazenes offered some promise for production ofboron nitride fibers.

Numerous references to the conversion of α-boron nitride to β-boronnitride have appeared and the vast majority depend upon the hightemperature-high pressure recrystallization of α-boron nitride preparedfrom classical thermal routes, e.g., pyrolysis of boric acid and urea.(See Moore, U.S. Pat. No. 3,578,403, entitled RECRYSTALIZATION OFPYROLYTIC BORON NITRIDE, to Zhdanovich, and U.S. Pat. No. 4,361,543,entitled PROCESS FOR PRODUCING POLYCRYSTALS OF CUBIC BORON NITRIDE, toZhdanovich, et al.). In a different approach, Beale, in U.S. Pat. No.4,655,893. entitled CUBIC BORON NITRIDE PREPARATION UTILIZING A BORONAND NITROGEN BEARING GAS, reported formation of β-boron nitride byactivated reactive evaporation of borazene (HNBH)₃ and a metal (Cr, Ni,Co. Al, Mu) catalyst.

In 1985, Hirano, et al., in U.S. Pat. No. 4,545,968, entitled METHODSFOR PREPARING CUBIC BORON NITRIDE SINTERED BODY AND CUBIC BORON NITRIDE,AND METHOD FOR PREPARING BORON NITRIDE FOR USE IN THE SAME, and U.S.Pat. No. 4,590,034, entitled METHOD FOR PREPARING SINTERED BODYCONTAINING CUBIC BORON NITRIDE AND METHOD FOR PREPARING CUBIC BORONNITRIDE, described very generalized high temperature - high pressureroutes to β-boron nitride involving borazene and substituted borazenes.

Specific polymeric boron-nitrogen compounds have been recently developedas pyrolysis precursors to boron nitride. One such process uses thetrichloroborazene, Cl₃ B₃ N₃ H₃, as the primary monomer as follows:##STR1## wherein Et₂ O represents diethyl ether, and n represents aninteger. This polymer is then pyrolized to produce solid powder α-boronnitride, as follows: ##STR2## This powder cannot be successfully usedfor a coating requiring liquid properties. Prior art publicationsdescribing this work include: Narula, et al., "Synthesis of BoronNitride Ceramics From Poly(borazinylamine) Precursors," Journal ofAmerican Chemical Society, (1987); "Precursors to Boron-NitrogenMacromolecules and Ceramics," Mat. Res. Soc. Symp. Proc. Vol. 73, p. 383(1986); and "New Precursors to Boron-Nitrogen Macromolecules andCeramics," Abstr. H6.4 Mat. Res. Soc. Meeting, Spring 1986.

Paine, et al., in U.S. patent application Ser. No. 07/312,956, entitledPRECURSORS FOR BORON NITRIDE COATINGS filed on even date herewith,describes the application of boron nitride coatings on various articles.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a compositionand method using such composition for pyrolytic conversion of thecomposition to boron nitride. The composition comprises aB-aminoborazene compound having a general formula selected from thecompounds consisting of (ClB)₂ (BNR₂)(NH)₃, (ClB)₂ [BN(H)R](NH)₃, or(ClB)₂ [BN(SiR₃)₂ ](NH)₃, wherein R may comprise an alkyl group or anaryl group. The B-aminoborazene compound may comprise a monoalkylamido,dialkylamido, monoarylamido, diarylamido, or disilylamido group. The Ralkyl groups preferably comprise methyl, ethyl, n-propyl, iso-propyl,iso-butyl, n-butyl, t-butyl, n-pentyl, or cyclo-hexyl, and the R arylgroups preferably comprise phenyl, tolyl, or mesityl.

The composition preferably comprises a solvent, which may be an organicsolvent comprised of the group consisting of chlorocarbons, ethers,organoacetates, arenes, and hydrocarbons. The composition may furthercomprise at least one cross-linking agent, which is preferably asilylamine compound of the general formula [(CH₃)₃ Si]₂ NR, wherein Rrepresents hydrogen or methyl. The silylamine compound preferablycomprises hexamethyldisilizane, [(CH₃)₃ Si]₂ NH orheptamethyldisilizane, [(CH₃)₃ Si]₂ N(CH₃), in an approximatelyequimolar amount to the B-aminoborazene compound. The composition of thepreferred embodiment may further comprise a dopant.

One preferred B-aminoborazene compound is2-B-dimethylamino-4,6-B-di-chloroborazene. This compound may be treatedwith borane(3) tetrahydrofuran complex for cleaving amino groups fromthe B-aminoborazene compound and replacing them with hydrogen.

The invention further comprises a process for the pyrolytic conversionof a polymeric precursor composition to boron nitride comprising thesteps of:

(a) obtaining a B-aminoborazene compound, as set forth above;

(b) dissolving the B-aminoborazene compound in a solvent in the presenceof a cross-linking agent to form a polymeric gel; and

(c) converting the polymeric gel to boron nitride by pyrolysis.

Solvents and cross-linking agents, useful in accordance with theinvention, are discussed above.

In the preferred embodiment, the polymeric gel is preferably heated atapproximately the solvent reflux temperature and the solvent issubstantially removed prior to step (c). The solvent is substantiallyremoved by a method such as decantation, vacuum evaporation, sol-geltechniques, or aerogel techniques.

The polymeric gel is pyrolyzed in step (c) at a sufficient temperatureand pressure for a sufficient time to obtain boron nitride. Thepyrolysis temperature is preferably between approximately 300° C. and900° C. to obtain amorphous boron nitride. The amorphous boron nitridemay be further pyrolyzed at a higher temperature to obtain crystallinehexagonal boron nitride, or the polymeric gel may be directly convertedto crystalline hexagonal boron nitride. A temperature of at least 1200°C. is preferred to obtain crystalline hexagonal boron nitride. Thepolymeric gel or the amorphous or hexagonal boron nitride may bepyrolyzed, preferably in the presence of a catalyst added to thereaction mixture, to obtain crystalline cubic boron nitride. Dopants maybe added to the polymeric gel.

The polymeric gel can be applied as a liquid coating on materials suchas oxides, non-oxides, metals, and/or glasses, and on forms such assubstrates, powders, fibers, crystals, and preformed parts. If desired,the resulting boron nitride product may be free of carbon impurities.

It is a primary object of the present invention to provide boron nitrideprecursor compositions which are useful for forming boron nitrideproducts and articles coated with boron nitride.

It is another object of the present invention to provide processes formaking boron nitride precursor compositions, which are easy andinexpensive.

Yet another object of the present invention is to provide coatingcompositions which are easy to apply to articles and convert to boronnitride.

Other objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, and in part will become apparent tothose skilled in the art upon examination of the following, or may belearned by practice of the invention. The objects and advantages of theinvention may be realized and attained by means of the instrumentalitiesand combinations particularly pointed out in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention provides several compositions and methods forproducing boron nitride, and includes boron nitride products,substrates, and articles coated with these compositions which convert toboron nitride. The invention is useful in the production of amorphous(non-crystalline) boron nitride, crystalline alpha (also designated ashexagonal and α) boron nitride, and crystalline beta (also designated ascubic and β) boron nitride.

The trichloroborazene monomer (HNBCl)₃, has three identical reactionsites suitable for borazene ring cross-linking reactions which wouldresult in B-N-B bridges, namely the three B-Cl bonds as shown. ##STR3##In the present invention, one of the chlorine substituents in thetrichloroborazene monomer is replaced by at least one dialkylamido ordiarylamido group, represented by the formula NR₂ ; or monoalkylamido ormonoarylamido group, represented by the formula (N)(H)(R); ordisilylamido group, represented by the formula (N)(R₃ Si)₂. Alkylgroups, R, particularly useful for the dialkylamido and monoalkylamidogroups of the invention, include but are not limited to the following:methyl (Me). CH₃ ; ethyl (Et). C₂ H₅ ; n-propyl (n-Pr), n-C₃ H₇ ;iso-propyl (i-Pr), i-C₃ H₇ ; iso-butyl (i-Bu), i-C₄ H₉ ; n-butyl (n-Bu),n-C₄ H₉ ; t-butyl (t-Bu), t-C₄ H₉ ; n-pentyl, n-C₅ H₁₁ ; andcyclo-hexyl, c-C₆ H₁₁. Aryl groups, R, particularly useful for thediarylamido and monoarylamido groups of the invention, include but arenot limited to the following: phenyl (Ph), C₆ H₅ ; tolyl, (CH₃)C₆ H₄ ;and mesityl, (CH₃)₂ C₆ H₃. Alkyl and aryl groups, R, useful in thedisilylamido groups of the invention, include but are not limited tomethyl, ethyl, and phenyl.

The resulting B-aminoborazene compounds are represented by the followinggeneral formulas: ##STR4## wherein R is an alkyl or aryl group, asdiscussed above.

The B-aminoborazene compounds are cross-linked to form a preceramicpolymer, which when pyrolized, yields boron nitride. The aminosubstituent group on one boron atom serves to limit cross-linkingreactions to the remaining two B-Cl functional groups and the productsare linear polymers. As used throughout the specification and claims,the term "polymer" is also intended to include "oligomer" and means aplurality of monomer units.

One preferred cross-linking agent, useful in accordance with theinvention for cross-linking with the B-aminoborazene compounds of theinvention, comprises a silylamine, having the general formula [R'₃ Si]₂NR", wherein R' represents methyl, ethyl, and phenyl, and R" representsan alkyl or aryl group, such as discussed above. The preferredsilylamines are hexamethyldisilizane, [(CH₃)₃ Si]₂ NH, andheptamethyldisilizane, [(CH₃)₃ Si]₂ N(CH₃). These cross-linking agentsare combined with the B-aminoborazene compounds, preferably in anequimolar amount with the B-aminoborazene compounds, and preferably inthe presence of an organic solvent. Preferred organic solvents, usefulin accordance with the invention, include but are not limited tochlorocarbons, ethers, organo acetates, arenes, and hydrocarbons, forexample, chlorobenzene, chloroform, methylene chloride, diethyl ether,tetrahydrofuran, ethyl acetate, amyl acetate, benzene, toluene, andhexane.

In one preferred method of the invention, the B-aminoborazene compounds,such as 2-B-dimethylamino-4,6-B-dichloro borazene, are dissolved in anorganic solvent, such as diethyl ether (Et₂ O), and a cross-linkingagent, such as hexamethyldisilizane, is added to the solution preferablyin an approximate equimolar amount with the borazene, as follows:##STR5## The resulting solution is preferably heated to reflux thesolvent and the reflux is continued for a sufficient time (e.g., atleast four hours) to ensure complete condensation of the borazenemonomer with the cross-linking agent. This results in the formation ofborazene gels. When the solvent and the borazene gels are heated at thereflux temperature (approximately the boiling point of the solvent), thegelation rate and the chain growth accelerates, and a more dense gel,i.e., the "n" number is larger, is obtained. The resulting borazene gelproducts are useful as polymeric precursors for pyrolytic conversion toboron nitride. The gelation rate depends upon (and is controlled bymodification of) the R (alkyl or aryl) group on the B-aminoborazene,substituent groups on the cross-linking agent, the solvent, and thetemperature. As can be appreciated by those skilled in the art, theabove formula is intended to include all B-aminoborazene compounds ofthe invention, as set forth above, and is not limited to the particularcompound shown immediately above in the formula. Likewise, the resultinggel in accordance with the invention are not limited to the resultinggel compound shown immediately above. These gels will differ dependingon the starting B-aminoborazene compounds and the varying processingcontemplated by this invention.

The polymeric precursor gels can be further processed to substantiallyremove the solvent by utilizing decantation, vacuum evaporation, sol-geland aerogel (critical-point drying) techniques, common to the art, forprocessing other types of gels. The sol-gel and aerogel techniques havebeen used extensively for the formation of coatings and films in glass(SiO₂) technology, but they have not been used extensively in the artfor non-oxide ceramic processing. Sol-gel processing can be accomplishedwith the B-aminoborazene compounds of the invention, cross-linkinggroups, and most organic solvents. The gels can take the form of theircontainers. In aerogel techniques, the gels may be extracted with liquidCO₂ under critical point conditions.

The gels may be pyrolyzed at a sufficient temperature and pressure for asufficient time to obtain boron nitride. Complete pyrolysis occurspreferably in a temperature range of between approximately 300° C. to900° C. for between approximately thirty minutes to twelve hours. Thepyrolysis is preferably performed under flowing nitrogen gas and withcontinuous removal of substantially all volatile products (e.g., (Me₃SiCl)) in vacuo, such as by vacuum evaporation with the resultingformation of amorphous boron nitride. The gels may be additionallypyrolyzed at higher temperatures of approximately 1200° C. or higher forbetween approximately thirty minutes to twelve hours, preferably underinert or non-reactive gases, such as nitrogen and argon, or reactivegases, such as air and ammonia, with the resulting formation ofcrystalline α-boron nitride (or β-boron nitride, discussed below). Theresulting boron nitride material may be contaminated with small amountsof carbon, for instance if the pyrolysis is accomplished in air.

In an alternative process to prevent any carbon from being present inthe resulting boron nitride products of the invention, the carbonoriginating from alkyl and aryl groups or amine nitrogen atoms can beremoved early in the chemical process (prior to extensive solventreflux) by treatment of the polymeric starting gel solution with, forexample, a borane(3) tetrahydrofuran complex, e.g., H₃ B.OC₄ H₈. Thisserves to cleave the amino groups discussed above (and representedgenerally by NR₂ in the following formula) and replace them withhydrogen substituents, as shown by the following: ##STR6## As can beappreciated by those skilled in the art, the NR₂ group shown in theabove formula can also be replaced by NRH, and N(SiR₃)₂. Pyrolysisresults in a boron nitride ceramic product completely free of carbonimpurities. In addition, pyrolysis gives a very high ceramic yield. Thepyrolysis preferably occurs at 300° C. to 900° C. where amorphous boronnitride is formed. Further heating at 1200° C. or higher producescrystalline α-boron nitride. Conversion to β-boron nitride may beaccomplished as discussed below.

If carbon is desired in the resulting boron nitride product, the carboncan be included or added to the B-aminoborazene monomer gel precursor,such as by alkyl or aryl group substitution on the borazene ringnitrogen atoms. Alternatively, the composition and process of theinvention allows for the production of carbon free boron nitride, suchas discussed above.

The boron nitride obtained in the procedure described above may beconverted to cubic boron nitride by any of the standard hightemperature/high pressure methodologies known in the art. Catalysts aretypically mechanically mixed with boron nitride in the art to convert itto cubic boron nitride. One advantage of the present invention is thatcatalysts may be introduced homogeneously to the gels. This in turnresults in homogeneous inclusion of the catalyst in the boron nitridematrices which may provide lower α-boron nitride to β-boron nitridetransformation conditions.

Dopants, such as phosphorus, sulfur, silicon, and other main group andtransition metal elements, may be incorporated in the starting solutionby introducing the dopant elements in the borazene ring or in thecross-linking agent and these are retained, in some cases, in the finalceramic product in a highly dispersed state. Dopants modify theproperties of the boron nitride in specific ways.

The gels may be used to coat metal oxides or inorganic oxides, forexample, alumina (Al₂ O₃), zirconia (ZrO₂), and magnesia (MgO), otheroxides, and non-oxides, in any forms or substrates, including singlecrystals, powders, and preformed parts, and fibers. The gels aredispersed on the oxide and non-oxide forms and then heated. When thegels on the forms are heated to approximately 1200° C. or higher, a thincoating (10Å to 1000Å or more) of highly crystalline α-boron nitrideforms on the oxide or non-oxide. This coating provides a protectivecoating on the oxide form or substrate.

One advantage of using the foregoing compositions and methods of theinvention, as compared to prior art processes, is that the gels haveliquid properties which allow them to be applied as films or coatings onoxide, non-oxide, glass and metal substrates, as well as other forms.This compares to the prior art in which vapor deposition processes areused; such processes are limited in application by the few useful gasphase reagents available. In addition, the high temperature stability offilms produced by vapor deposition techniques is less than the stabilityof the coatings produced by this method. Articles requiring a boronnitride coating can be coated, in accordance with the invention.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE I

In the first stage, 9.9 g of 2,4,6-B-trichloroborazene was dissolvedunder dry non-reactive gas in diethyl ether (200 ml) and dimethylamine(4.86 g) was condensed into the reaction mixture at -78° C. Aftercomplete addition, the reaction mixture was stirred and warmed to 25° C.The resulting mixture was stirred for 8-12 hours and then filtered. Theresidue from the filtration (predominantly Me₂ NH₂ Cl) was washed twicewith diethyl ether and the combined ether filtrates were evaporated todryness. The resulting residue was dissolved in hexane (200 ml) andfiltered to remove any Me₂ NH₂ Cl carried through by the ether. Thehexane filtrate was vacuum evaporated and the remaining product (Me₂NB)(BCl)₂ (NH)₃ was purified by sublimation. The yield was 9.1 g (87%).

In the second stage, the resulting product.2-B-dimethylamino-4,6-B-di-chloroborazene (6.9 g) was dissolved inchlorobenzene (approximately 50 ml) and hexamethyldisilizane (5.76 g)was added with stirring. After complete addition the stirring wasstopped, and a colorless gel formed. The reaction mixture was thenheated at chlorobenzene reflux (boiling point) for approximately 5-6hours to form a denser gel.

Samples of the preceramic borazinyl polymer obtained by sol-geltechniques and by CO₂ extraction (aerogel conditions) were pyrolyzed invacuo between 600° C. and 900° C. for 12 hours. The resulting ceramicproducts contained carbon and were amorphous. Subsequent heating to1200° C. for two hours in air produced a grey α-boron nitride samplecontaminated with small amounts of carbon. Alternatively, the ceramicproduct was pyrolyzed under NH₃ for 12 hours and the final product wasfree of carbon impurities.

EXAMPLE II

A borazinyl gel sample was obtained by combining2-B-dimethylamino-4,6-B-dichloroborazene (7.8 g) andhexamethyldisilizane (6.86 g) in 200 ml diethyl ether. The remainingliquid after gelation was drained from the reaction vessel, and the gelwas then exposed to fresh diethyl ether (approximately 150 ml). To thismixture was added H₃ B.OC₄ H₈ (85 ml), 1M solution in hexane). Themixture was allowed to stand for twenty-four hours and then the liquidcovering the gel was removed. ¹¹ B NMR analysis of the liquid showed thepresence of no H₃ B.OC₄ H₈, but formation of Me₂ NB₂ H₅ and Me₂ N(H)BH₃.

The remaining gel was dried in vacuo and pyrolyzed in vacuo between 600°C. and 900° C., leaving amorphous white boron nitride. Treatment of thissolid for 12 hours at 1200° C. in N₂, air or NH₃, resulted incrystalline α-boron nitride, free of carbon.

Although the invention has been described with reference to thesepreferred embodiments, other embodiments can achieve the same results.Variations and modifications of the present invention will be obvious tothose skilled in the art and it is intended to cover in the appendedclaims all such modifications and equivalents.

What is claimed is:
 1. A process for the pyrolytic conversion of apolymeric precursor composition to boron nitride comprising thefollowing steps:(a) obtaining a B-aminoborazene compound having ageneral formula selected from the group consisting of (ClB)₂(BNR₂)(NH)₃, (ClB)₂ [BN(H)R](NH)₃ and (ClB)₂ [BN(SiR₃)₂ ](NH)₃ andwherein each R is a member selected from the group consisting of alkylgroups and aryl groups; (b) dissolving the B-aminoborazene compound in asolvent in the presence of a cross-linking agent to form a polymeric gelby linear polymerization; (c) removing substantially all the solventfrom said polymeric gel; and (d) converting the polymeric gel to boronnitride by pyrolysis.
 2. The process of claim 1 wherein theB-aminoborazene compound has the general formula (ClB)₂ (BNR₂)(NH)₃, andwherein NR₂ is dialkylamido.
 3. The process off claim 2 wherein each Ris a member selected from the group consisting of methyl, ethyl,n-propyl, iso-propyl, iso-butyl, n-butyl, t-butyl, n-pentyl, andcyclo-hexyl.
 4. The process of claim 1 wherein the B-aminoborazenecompound has the general formula (ClB)₂ [BN(H)R](NH)₃, and wherein theN(H)R is monoalkylamido.
 5. The process of claim 4 wherein each R is amember selected from the group consisting of methyl, ethyl, n-propyl,iso-propyl, iso-butyl, n-butyl, t-butyl, n-pentyl, and cyclo-hexyl. 6.The process of claim 1 wherein the B-aminoborazene compound has thegeneral formula (ClB)₂ (BNR₂)(NH)₃, and wherein NR₂ is diarylamido. 7.The process of claim 6 wherein each R is a member selected from thegroup consisting of phenyl, tolyl, and mesityl.
 8. The process of claim1 wherein the B-aminoborazene compound has the general formula (ClB)₂[BN(H)R](NH)₃, and wherein N(H)R is monoarylamido.
 9. The process ofclaim 8 wherein R is a member selected from the group consisting ofphenyl, tolyl, and mesityl.
 10. The process of claim 1 wherein theB-aminoborazene compound has the general formula (ClB)₂ [BN(SiR₃)₂](NH)₃, and wherein N(SiR)₃)₂ is disilylamido.
 11. The process of claim10 wherein R comprises at least one member selected from the groupsconsisting of methyl, ethyl, and phenyl.
 12. The process of claim 1wherein the cross-linking agent comprises a silylamine compound.
 13. Theprocess of claim 1 wherein prior to step (d), the polymeric gel isheated at approximately the solvent reflux temperature.
 14. The processof claim 1 wherein the solvent is substantially removed by at least onemethod selected from the group consisting of decantation, vacuumevaporation, sol-gel techniques, and aerogel techniques.
 15. The processof claim 1 wherein the polymeric gel is pyrolyzed in step (d) at asufficient temperature and pressure for a sufficient time to obtainamorphous boron nitride.
 16. The process of claim 15 wherein thepolymeric gel is pyrolyzed in step (d) at a temperature of betweenapproximately 300° C. and 900° C. to obtain amorphous boron nitride. 17.The process of claim 16 wherein the amorphous boron nitride is furtherpyrolyzed at a sufficient temperature and pressure for a sufficient timeto obtain crystalline hexagonal boron nitride.
 18. The process of claim1 wherein the polymeric gel is pyrolyzed in step (d) at a sufficienttemperature and pressure for a sufficient time to obtain crystallinehexagonal boron nitride.
 19. The process of claim 18 wherein thepolymeric gel is pyrolyzed in step (d) at a temperature of at least1200° C. to obtain crystalline hexagonal boron nitride.
 20. The processof claim 1 wherein the polymeric gel is pyrolyzed in step (d) at asufficient temperature and pressure for a sufficient time to obtaincrystalline cubic boron nitride.
 21. The process of claim 20 wherein thepolymeric gel is pyrolized in step (d) in the presence of a catalyst toobtain crystalline cubic boron nitride.
 22. The process of claim 21wherein a catalyst is added to the polymeric gel prior to step (d). 23.The process of claim 1 wherein, prior to step (d), at least one dopantis added to the polymeric gel.
 24. The process of claim 1 wherein theresulting boron nitride product is free of carbon impurities.
 25. Theprocess of claim 1 wherein the pyrolysis in step (d) is conducted in theabsence of air.
 26. The process of claim 24 wherein the pyrolysis instep (d) is conducted in the absence of air.
 27. The process of claim 1wherein the pyrolysis is conducted in the presence of at least one gasselected from the group consisting of N₂, NH₃ and an inert gas.
 28. Theprocess of claim 1 wherein the solvent in step (b) comprises at leastone member selected from the group consisting of chlorocarbons, ethers,organoacetates, arenes, and hydrocarbons.